In various parts of Great Britain and Ireland we meet with old volcanic rocks,—lavas, intrusive dykes, and sheets of basalt, etc., together with vast deposits of volcanic ash, which, sinking into the old neighbouring seas, became stratified, or arranged in layers like the ordinary sedimentary rocks. In some cases we see embedded in these layers the very "bombs" that were thrown out by the old volcanoes (see page [253]). And besides these purely volcanic rocks, we often meet in these areas with great bosses of granite, which must have been in some way connected with the old volcanoes, and probably were in many cases the source from which much of the volcanic rock was derived. But more than this, in a few instances we have the site of the old volcano itself marked out by a kind of pipe, or "neck," now filled with some of its volcanic débris in the shape of coarse, rounded fragments (see page [277]).
During a very ancient period, known to geologists as the Silurian Period, great lava-flows took place from volcanoes situated where North and South Wales and the Lake District now are; and by their eruptions a vast amount of volcanic ash was made, which fell into the sea and slowly sank to the bottom, so that the shell-fish living there were buried in the strata thus formed, and may now be seen in a fossilised condition.
Fig. 1. THE RANGES OF THE GREAT BASIN, WESTERN STATES OF NORTH AMERICA, SHOWING A SERIES OF
GREAT FRACTURES AND TILTED MASSES OF ROCK.
Fig. 2. SECTION THROUGH SNOWDON.
Thus Snowdon, Cader Idris, the Arans, Arenig Mountain, and others, are very largely made up of these ancient volcanic materials. The writer has picked up specimens of fossil shell-fish near the summit of Snowdon from a bed of fine volcanic ash that forms the summit. Fig. 2 represents a section through Snowdon, from which it will be seen that we have first a few sedimentary strata, S, then a great lava-flow, L; and that volcanic ashes accumulated on the top of this, of which A A are patches still left. B is an intrusive dyke of a basaltic rock that forced its way through afterwards. Again, in the Lake District there is a well-known volcanic series of stratified rocks of the same age, consisting mostly of lavas and ashes, the total thickness of which is about twelve thousand feet (known as the "Green Slates and Porphyries"), so that a large part of some of the mountains there have also been built up by volcanic action; but no traces of the old volcanoes remain.
Going farther north we find abundant proof that volcanic action on a prodigious scale took place in Scotland during the very ancient period of the Old Red Sandstone, with which the name of Hugh Miller will always be associated. In Central Scotland we see lava-flows and strata formed of volcanic ash, with a thickness of more than six thousand feet, fragments of which, having escaped the destructive agents of denudation, now form important chains of hills, such as the Pentland, Ochil, and Sidlaw ranges. Nor was the volcanic action confined to this region. In the district of the Cheviot Hills similar volcanic rocks are to be seen. But here again the old volcanoes have long since been swept away, leaving us only portions of their outpourings buried in the hills.